The term “biomass” generally refers to a total amount of organisms that inhabit or exist in a given area. When such term is used with regard to plants, in particular, it refers to dry weight per unit area. Biomass units are quantified in terms of mass or energy. The expression “biomass” is synonymous with “seibutsutairyo” or “seibutsuryo.” In the case of plant biomass, the term “standing crop” is occasionally used for “biomass.” Since plant biomass is generated by fixing atmospheric carbon dioxide with the use of solar energy, it can be regarded as so-called “carbon-neutral energy.” Accordingly, an increase in plant biomass is effective for the preservation of the global environment, the prevention of global warming, and mitigation of greenhouse gas emissions. Thus, technologies for increasing the production of plant biomass have been industrially significant.
Plants are cultivated for the purpose of using some tissues thereof (e.g., seeds, roots, leaves, or stems) or for the purpose of producing various materials, such as fats and oils. Examples of fats and oils produced from plants that have heretofore been known include soybean oil, sesame oil, olive oil, coconut oil, rice oil, cottonseed oil, sunflower oil, corn oil, safflower oil, palm oil, and rapeseed oil. Such fats and oils are extensively used for household and industrial applications. Also, fats and oils produced from plants are used for biodiesel fuel or bioplastic raw materials, and the applicability thereof for alternative energy to petroleum is increasing.
In particular, an energy crop such as sugarcane can be used as a raw material for biofuel. Thus, the increased production of the total mass of a plant itself (the amount of plant biomass) is expected. Under such circumstances, improvement in productivity per unit of cultivation area is required in order to increase the amount of plant biomass production. It has been found that, if the number of cultivated plants is assumed to be constant per unit of cultivation area, improvement in the amount of biomass per plant would be necessary.
However, it is considered that, since many genes are related to the amount of plant biomass (a so-called “kind of quantitative trait”), individual gene introduction, deletion, or modification is insufficient for effectively increasing the production of plant biomass. For example, U.S. Pat. No. 7,834,146 discloses a technique comprising introducing one or more polypeptides selected from among approximately 180 exemplified polypeptides into a plant (i.e., activation), thereby improving the efficiency of a plant in terms of nitrogen use and increasing biomass production. Such approximately 180 kinds of polypeptides contain clathrin-associated protein complex small subunits (yeast AP-2; Yjr058c). However, there has been no disclosure of evidence demonstrating the effects of the clathrin-associated protein complex small subunits for increasing biomass production.
Vesicular transport is a mechanism for intracellular or extracellular transportation of a substance through a vesicle. A wide variety of substances, including proteins and lipids, are transported through vesicles. In general, it is known that inhibition of intracellular vesicular transport leads to an increase in the size of a cell, although the biomass amount is small (Tahara et al., 2007, Clathrin is involved in organization of mitotic spindle and phragmoplast as well as in endocytosis in tobacco cell cultures, Protoplasma, 230: 1-11). Also, Andersson, M. X. and Sandelius, A. S., 2004, A chloroplast-localized vesicular transport system: A Bioinformatics Approach, BMC Genomics, 5: 40 describes that proteins associated with transportation to the chloroplast thylakoid membrane can be predicted via bioinformatics analysis, and it lists genomic homologs of Arabidopsis thaliana associated with proteins associated with membrane transportation in yeast identified via homology analysis. According to Andersson, M. X. and Sandelius, A. S., 2004, A chloroplast-localized vesicular transport system: A Bioinformatics Approach, BMC Genomics, 5: 40, proteins homologous to yeast Ret3 are At3g09800 and At4g08520 of Arabidopsis thaliana. 
No plants derived from Arabidopsis thaliana through At3g09800 overexpression or deletion have been known. However, U.S. Pat. Nos. 7,834,146, 7,214,786, 8,299,318, 7,569,389, and WO 2009/037,279 disclose that biomass production can be increased through overexpression of a gene encoding a protein having, for example, approximately 70% or higher sequence similarity to a protein encoded by At3g09800.